In response to partial denervation, it is now well-known that new synapses form to replace those lost. Little, however, is known about the rules which determine which new synapses form and where they form. Our experiments have focused on the septohippocampal pathway. It appears as if the development of septal lamination and plasticity of its connections in response to lesions can be explained in terms of the growth patterns of hippocampal CA4 fibers. It further appears that transmitter identity of the septal fibers is more critical for pattern formation than the source of the tissue. Our work is continuing to test this hypothesis. Our main approach is to use implants of various brain tissues which employ the same transmitter but are of different origin.